Currently, in a direct current power supply input system or a related electronics system, to prevent negative surge from damaging a next circuit and an equipment, a surge protection circuit is set between a direct current input and the next circuit, to cut off power supply to the next circuit timely in the case of negative surge, discharge a surge current using a varistor resistor (VR) or a transient voltage suppressor (TVS), so as to achieve surge protection for the next circuit.
Specifically, as shown in FIG. 1(a), a surge protection circuit includes a cutoff circuit and a discharge circuit, where the cutoff circuit is implemented as follows in detail: a diode D1 and a diode D2 are set at the positive end and negative end of a direct current input, respectively, the anode of D1 is connected to the positive end of the direct current input, and the cathode of D2 is connected to the negative end of the direct current input; a capacitor C is set between the diodes D1 and D2 and a next circuit, and two ends of the capacitor C are connected to the cathode of D1 and the anode of D2, respectively. Of course, as shown in FIG. 1(b), diodes D1 and D2 of the cutoff circuit may also be replaced by field-effect transistors M1 and M2. The discharge circuit is a VR or TVS connected between the positive end and the negative end of the direct current input.
A working principle of a surge protection circuit in the prior art is: in the case of normal working, diodes (or field-effect transistors) of a cutoff circuit are in a forward conductive status, and a current flows from the positive end of direct current input, through a next circuit, to the negative end of the direct current input. When negative surge occurs, the current flows from the negative end to the positive end temporarily; at this time, D1 and D2 (or M1 and M2) are cut off reversely, to achieve a cutoff function and protect the next circuit. A VR can adjust resistance according to a voltage and discharge a surge current; a TVS can convert the surge current into heat for dissipation to achieve a discharge purpose.
However, taking FIG. 1(a) as an example, after diodes D1 and D2 are cut off, they withstand a high reverse voltage (the reverse voltage is equal to a sum of a clamping voltage of a varistor resistor and a voltage of a capacitor C). For example, for the direct current input ranging from 38.4 V to 72 V, because the clamping voltage of the varistor resistor is as high as about 130 V, D1 and D2 withstand a reverse voltage that is normally above 170 V. Therefore, a diode or field-effect transistor of a cutoff circuit may only be a model with a high specification; a high clamping voltage seriously restricts model selection of an electronic equipment, and the high clamping voltage easily causes diodes D1 and D2 (or field-effect transistors M1 and M2) to lose effect or be damaged in the case of negative surge.